Of the xylene isomers, para-xylene is of particular value since it is useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers. Equilibrium mixtures of xylene isomers either alone or in further admixture with ethylbenzene generally contain only about 22-24 wt % para-xylene and separation of para-xylene from such mixtures typically requires superfractionation and multistage refrigeration steps. Such processes involve high operational costs and result in only limited yields. There is therefore a continuing need to provide processes for producing xylenes which are highly selective for para-isomer.
One known method for producing xylenes involves the alkylation of toluene with methanol over a solid acid catalyst. Thus the alkylation of toluene with methanol over cation-exchanged zeolite Y has been described by Yashima et al. in the Journal of Catalysis 16, 273-280 (1970). These workers reported selective production of para-xylene over the approximate temperature range of 200 to 275° C., with the maximum yield of para-xylene in the mixture of xylenes, i.e., about 50% of the xylene product mixture, being observed at 225° C. Higher temperatures were reported to result in an increase in the yield of meta-xylene and a decrease in production of para and ortho-xylenes.
More recently, selectivities to para-xylene in excess of 90 wt % (based on total C8 aromatic product) have been reported by reacting toluene with methanol in the presence of a catalyst comprising a porous crystalline material having a Diffusion Parameter for 2,2 dimethylbutane of about 0.1-15 sec−1 when measured at a temperature of 120° C. and a 2,2 dimethylbutane pressure of 60 torr (8 kPa). The porous crystalline material is preferably a medium-pore zeolite, particularly ZSM-5, which has been severely steamed at a temperature of at least 950° C. in the presence of at least one oxide modifier, preferably including phosphorus, to control reduction of the micropore volume of the material during the steaming step. See U.S. Pat. Nos. 6,423,879 and 6,504,072.
In contrast, U.S. Pat. No. 7,399,727 reports that improved selectivity in the methylation of toluene to para-xylene can be achieved through the use of a catalyst comprising a phosphorus-containing ZSM-5-type zeolite having a silica/alumina molar ratio of at least 200, a phosphorus content of at least 8% by weight of zeolite and multiple phosphorus species exhibited by multiple 31P MAS NMR peaks with maxima at from about 0 to about −50 ppm. The phosphorus-containing ZSM-5 may be used without steaming or may be steamed at low or mild temperatures, such as from about 150° C. to about 350° C. In addition, the phosphorus-containing ZSM-5 may be used with or without a binder.
According to the present invention, it has now been found that certain severely steamed, phosphorus-containing, bound ZSM-5 catalysts show a unique combination of selectivity and stability when used in the methylation of toluene and/or benzene to para-xylene at high steam partial pressures. Since water is an inevitable by-product of the methylation process, the ability of the catalyst to retain its selectivity over long periods of time at high steam partial pressures represents a significant advantage over existing processes.